Prenyl chloride is an important intermediate in the preparation of valuable aroma chemicals such as linalool, geraniol, citronellol as well as many other aroma chemicals which are derived from these compounds. The thus prepared aroma products are themselves useful as components when compounding fragrant compositions of matter which are useful in the cosmetic or toiletry industry, said compositions being useful in perfumes, colognes, talcum powders, soaps, detergents, bath powders, etc. The prior art teaches procedures in which isoprene is reacted with anhydrous hydrogen chloride or aqueous concentrated hydrochloric acid. Generally speaking there are two methods which may be utilized in the preparation of the desired compound. One method involves effecting the reaction at temperatures below about 0.degree. C. with the attendant formation of the tertiary chloride derivative, namely, 3-chloro-3-methylbutene-1. Thereafter the temperature is raised to about 0.degree. C. to a range of from about 10.degree. to 30.degree. C. whereby the tertiary chloride product is isomerized to form 1-chloro-3-methylbutene-2 (prenyl chloride). Alternatively, the treatment of isoprene with hydrochloric acid or hydrogen chloride can be conducted at higher temperatures ranging from about 0.degree. to about 20.degree. C. to produce the prenyl chloride in a direct manner. For example, Britist Pat. No. 855,696 discloses the treatment of isoprene with concentrated aqueous hydrochloric acid at temperatures ranging from 10.degree. to 15.degree. C. However, the yield which is disclosed is 63% of prenyl chloride which distills over a 7.degree. range of from 106.degree. to 113.degree. C. In addition, there is also present 8% of the tertiary chloride compound. Another prior art reference, French Pat. No. 1,548,516, discloses the treatment of isoprene with concentrated hydrochloric acid and concentrated sulfuric acid at a temperature of 30.degree. C. In this instance there has been reported an 80% total yield of product consisting of 29.5% of unreacted isoprene, 6.8% of the tertiary chloride and only 57% of the desired compound, namely, prenyl chloride. Another prior art reference for the preparation of prenyl chloride is F. K. Sngryan et al, Arm. Khim. Zh., 26, 563 (1973) in which a 92% selectivity of prenyl chloride is reported at 20.degree. C. but only at a 50-55% conversion of isoprene. In general, the results reported in the other processes of prior art are of about the same order of magnitude as cited in the above.
In contradistinction to the methods which are set forth in the prior art, we have now discovered that by treating isoprene with concentrated hydrochloric acid as the hydrohalogenating reagent in the presence of an alkali or alkaline earth metal chloride compound and, in an advantageous manner, in the presence of the byproduct heads and tails fractions obtained from the distillation of a prior reaction involving the same compounds, it is possible to obtain high yields of prenyl chloride with a high selectivity of the desired compound and a high conversion of isoprene.
This invention relates to a process for the synthesis of prenyl chloride. More specifically, the invention is concerned with a process for the synthesis of prenyl chloride which involves treating isoprene with hydrochloric acid in the presence of an alkali or alkaline earth metal chloride compound whereby a high conversion and selectivity of the starting material and product is obtained plus an excellent purity of the product.
As hereinbefore set forth, a prenyl chloride, which also may be nomenclated as 1-chloro-3-methylbutene-2, is a valuable intermediate for the synthesis of terpenoid alcohols such as linalool, geraniol, citronellol, etc., the latter compounds forming components of aromatic compositions of matter. It is a desired feature of the process for the synthesis of prenyl chloride that the product be obtained in relatively high yields with a correspondingly high purity of the product. This high purity of the product is a necessary feature inasmuch as many impurities which may be present must be removed by costly distillation methods in order that the product may be utilized in the further formation of aroma chemicals which possess distinctive and desired fragrances. By utilizing the process of this invention, it is possible to attain these ends.
It is therefore an object of this invention to provide a process for the synthesis of prenyl chloride.
A further object of this invention is to provide a process for the preparation of prenyl chloride by treating isoprene with a hydrogen chloride compound in the presence of an alkali or alkaline earth metal chloride compound whereby the desired product is obtained in a high yield and will possess a high purity.
In one aspect an embodiment of this invention resides in a process for the preparation of prenyl chloride which comprises reacting isoprene with hydrochloric acid in the presence of an alkali metal chloride at reaction conditions, and recovering the resultant prenyl chloride.
A specific embodiment of this invention if found in a process for the preparation of prenyl chloride which comprises reacting isoprene with concentrated hydrochloric acid in the presence of sodium chloride at a temperature in the range of from 0.degree. to about 30.degree. C., and recovering the resultant prenyl chloride.
Other objects and embodiments will be found in the following further detailed description of the present invention.
As hereinbefore set forth in greater detail, the present invention is concerned with an improvement in the process for synthesizing prenyl chloride, said improvement resulting in the obtention of greater yields of purer product with correspondingly high percentage of selectivity. As opposed to many prior art processes which utilized gaseous or anhydrous hydrogen chloride, the present invention permits the use of concentrated aqueous hydrochloric acid which is a less expensive reagent than is the anhydrous hydrogen chloride. In addition, the use of the concentrated aqueous hydrochloric acid will permit a less complicated operating procedure to be employed. The synthesis of prenyl chloride according to the process of the present invention is effected by treating isoprene with aqueous concentrated hydrochloric acid and preferably a 37% concentrated hydrochloric acid solution although 32% acid may be used if desired. The improvement in the process lies in the presence of an inorganic chloride compound selected from the group consisting of alkali metal and alkaline earth metal chlorides. Specific examples of these chlorides will include solium chloride, potassium chloride, lithium chloride, rubidium chloride, cesium chloride, magnesium chloride, calcium chloride, strontium chloride, barium chloride, etc., the preferred chloride compounds comprising sodium chloride and potassium chloride and especially sodium chloride due to their relatively greater availability and correspondingly lower cost. In the preferred embodiment of the invention, the hydrochloric acid is present in a molar excess in a range of from about 1.1 to about 2.5 moles of hydrochloric acid per mole of isoprene. Generally speaking the hydrochloric acid and the sodium chloride are preferably added in an incremental manner to the reaction vessel during the reaction time which may range from about 0.5 to about 10 hours or more in duration. The concentrated hydrochloric acid and the sodium chloride in solid form may be added separately or, if so desired, the sodium chloride and hydrochloric acid may be admixed prior to being added to the reactor and the resultant mixture is then charged thereto in a single stream. By utilizing an alkali chloride compound such as sodium chloride, a saturated solution of the aqueous phase will be maintained during the course of the reaction thus facilitating the transfer of the hydrogen chloride from the aqueous phase to the organic phase. The treatment of the isoprene with the hydrochloric acid and the alkali metal chloride is accomplished at temperatures ranging from subambient (about 0.degree. C.) up to about 30.degree. C. or more, thus permitting the direct synthesis of prenyl chloride (1-chloro-3-methylbutene-2) rather than first forming the tertiary chloride compound (3-chloro-3-methylbutene-1) which must then be isomerized to form the desired compound.
In another embodiment of the invention, it is contemplated that the treatment of the isoprene with the hydrochloric acid and inorganic chloride compound may also be advantageously effected in the presence of byproduct heads and tails fractions which had been obtained from the distillation of the product according to a prior run. By recycling the heads and tails fractions from the distillation, it has been found that the formation of the tertiary chloride compound is minimized over the amount which was produced during the previous run. Likewise, it has also been found that the presence of the inorganic chloride compound also reduces the formation of 2,4-dichloro-2-methylbutane as well as the amount of heavies which have formed during the previous run. The advantage of utilizing the presence of the byproduct heads and tails fractions to produce the desired result will be graphically illustrated in the examples at the end of the specification.
As hereinbefore set forth, by utilizing the presence of an alkali metal chloride compound such as sodium chloride as well as in the presence of heads and tails fractions from a preceding distillation for the recovery of prenyl chlorides, it is possible to effect the reaction in either a batch or continuous manner of operation. For example, when a batch type operation is used, in one embodiment of the invention, a predetermined amount of isoprene is placed in a suitable reactor vessel along with, if so desired, the byproduct heads and tails materials from the distillation of a previous reaction. The reactor vessel is maintained at a predetermined temperature level which may range from about 0.degree. to about 10.degree. C. Thereafter the concentrated hydrochloric acid in a 37% concentration and solid alkali metal chloride such as sodium chloride are added in incremental portions during a period of about 2 hours. As hereinbefore set forth, the hydrochloric acid and solid sodium chloride may be added in separate streams or, if so desired, they may also be admixed prior to entry into said reactor vessel and the resulting mixture charged thereto in a single stream. At the end of the 2-hour period, during which moderate stirring is effected, the temperature of the reactor, which may be controlled by means of any external cooling means known in the art, is raised to a range of from about 15.degree. to about 20.degree. C. and maintained in this range for an additional period of 3 hours. At the end of the 3-hour period, the temperature is then lowered to about 0.degree. to 5.degree. C. and the bottom aqueous layer is separated from the organic layer after mixing is discontinued and discarded. The organic layer is then treated with a suitable alkaline drying agent such as sodium carbonate, potassium carbonate, lithium carbonate, calcium carbonate, etc., whereby the product is dried and, in addition, any hydrogen chloride which is dissolved therein will be neutralized. Following this, the organic layer is separated from the drying agent by filtration, decantation, etc., and subjected to fractional distillation, preferably under reduced pressure whereby the desired prenyl chloride is separated and recovered. The heads and tails fractions from this distillation may then be recycled and added to fresh isoprene in a subsequent run which is effected in a similar manner to that just described. By repeatedly recycling the byproducts comprising the heads and tails materials from one run to the next plus the addition of an alkali metal chloride to the reaction system, an approximately steady state system is achieved which will afford a relatively high yield of the desired product with a concurrent diminution of undesired tertiary chloride products such as 3-chloro-3-methylbutene-1, dichloro-substituted products such as 2,4-dichloro-2-methylbutane, and other heavies. These advantages will be hereinafter shown in greater detail in the examples which are appended to the specification.
It is also contemplated within the scope of this invention that the process for obtaining improved yields of prenyl chloride may also be effected in a continuous manner of operation. When such a type of operation is employed, a quantity of isoprene is continuously charged to a reactor vessel which is maintained at the proper operating conditions of temperature and pressure. In like manner, a mixture of the concentrated hydrochloric acid and an alkali metal chloride compound such as sodium chloride admixed therein is intermittently charged to the reaction vessel at a predetermined rate over a predetermined period of time whereby said isoprene is treated with hydrochloric acid to form the desired prenyl chloride. In addition, heads and tails materials which are obtained from a distillation of already formed product are also recovered from the distillation apparatus and charged to the reactor vessel concurrently with the isoprene. After completion of the desired residence time, the reactor effluent is continuously withdrawn and the product isolated as in the above-described batch process and passed to a distillation apparatus whereby the desired prenyl chloride is separated and recovered while the aforesaid heads and tails materials are recycled to form a portion of the feed stock.
In addition to the aforementioned steps of preparing and recovering the desired prenyl chloride, it is also comtemplated within the scope of this invention that the distillation of the product of the reaction may be effected over anhydrous alkaline metal carbonates such as sodium carbonate, potassium carbonate, lithium carbonate, etc., which will act as a stabilizer and prevent decomposition of the material during the distillation step.